Fluidization technology has been developed for close to one century from its first application to coal gasification by Winkler in the 1920s [1]. Since then, fluidized beds have been used for many different applications such as gas-solid, liquid-solid, and gas-liquid-solid contactors and to carry out a variety of different processes as chemical and biological reactors.
The application of fluidized beds to the biological wastewater treatment processes originated from observations of denitrification, made whilst using activated carbon to remove organic compounds from chemically treated sewage in a fluidized bed [2]. Since then, biological fluidized beds (BFBs) have been investigated for carbon oxidation, nitrification, denitrification and anaerobic treatment, for a wide variety of waters and wastewaters [3, 4, 5, 6]. In contrast to conventional biofilm reactors, in which media are fixed, the media immobilized on particles in a BFB are suspended in upflowing wastewater. Hence, BFBs have the advantages of increased liquid-solid interfacial area, enhanced contact between liquid and media, and increased liquid flowrate, as well as enhanced contact between gas and liquid if gas is present.
Biological fluidized beds are usually liquid-solid fluidized beds or gas-liquid-solid fluidized beds when air is added to the system for aerobic process. Fluidization of liquid-solid systems is controlled by the liquid flow rate [7, 8]. For a given liquid-solid system, the bed is initially fixed when liquid flow rate is lower than the minimum fluidization velocity. When liquid velocity exceeds the minimum fluidization velocity, the operation of the bed is transferred into the conventional or particulate liquid-solid fluidization regime (PFR). In a liquid-solid conventional or particulate fluidized bed (PFB), solid particles are nearly uniformly suspended in the liquid and are therefore in good contact with the liquid phase, with excellent interfacial mass transfer due to the continuous movement of the particles in the liquid and the drag exerted by the liquid. With a further increase of the liquid velocity, some particles begin to be transported out of the bed. At this time, the fluidized bed is in the transition from the particulate fluidization regime to the fast fluidization regime [7, 8, 9, 10].
When the liquid velocity is sufficiently high, normally when it is higher than the terminal velocity of the particles in the bed, large quantity of particles are transported out of the bed. At this point, the bed has entered the fast fluidization regime (FFR) (sometimes, also referred to as the circulating fluidization regime) to form a fast fluidized bed (FFB). (Sometimes, FFB is also referred to as a riser fluidized bed or simply riser, but the term riser may also be broadly used for any fluidized bed with a net solids upflow which is the definition we will adopt here in this application). Fast fluidized bed normally provides even higher liquid-solid mass transfer efficiency and higher liquid throughput than particulate fluidized bed. In FFB, as the particles are constantly carried out of the bed (normally from the top) by the upflowing liquid, more particles need to be fed into the FFB (normally at or near the bottom) to maintain a liquid-solid suspension. These particles fed into the FFB bottom may be fresh particles or particles from an upstream process in cases where the particles only need to go through the FFB once.
In cases where the particles should remain in the FFB for an extended period of time, particles flowing out from the top of the FFB should be recirculated back to feed into the bottom of FFB. Such recirculation may be through a standpipe for particle downflow or a conventional PFB where there is a net particle downflow, but can also be realized by other means. When such particle recirculation is realized through a particulate fluidized bed (PFB) with net particle downflow, the entire system including the FFB and the PFB forms a particle flow loop which is often referred to as a circulating fluidized bed (CFB).
In the case of gas-liquid-solid (three-phase) fluidized bed, air or other gases is injected into the bed, normally at or near the bottom of the bed. This applies to either a FFB or a PFB, or both. Gas passes through the bed as rising bubbles while interacting with the liquid and solid particles. Similar transitions occur from the fixed bed, to a conventional particulate gas-liquid-solid (three-phase) fluidized bed, and then to a fast gas-liquid-solid (three-phase) fluidized bed [11-13]. In a conventional particulate three-phase fluidized bed with bubbles flowing upwards, there are actually more than just one fluidization regime (such as dispersed bubbling and coalescing bubbling regimes) but for simplicity we will just refer them collectively as particulate fluidization regime (PFR) and the corresponding particulate three-phase fluidized bed as particulate fluidized bed (PFB).
The advantages of biological fluidized beds (BFBs) may be utilized to increase the efficiency of wastewater treatment processes. Traditionally, the most conventional and widely used design for municipal and industrial wastewater treatment is the activated sludge (AS) process, which employs a primary clarifier, an aeration tank, and a secondary clarifier. In such a process, nitrogen is merely converted from the more toxic ammonia form to nitrates and phosphorus removal is achieved by chemical addition. The process is classified as a suspended-growth system in which microbes responsible for treatment are in suspension. “Fixed-film” bioreactors have some advantages over the conventional activated sludge process. In fixed-film bioreactors, bacterial films are immobilized on an attachment media rather than remain in suspension. Due to the immobilization of biomass on media, the loss of biomass by shearing is the only mechanism for the escape of biosolids in the bioreactor effluent. The sloughed biomass is mostly decaying biomass that has good settling characteristics and can be readily separated from the liquid [14]. The most salient advantage of fixed-film vis-à-vis suspended growth systems is higher biomass densities per unit volume, resulting not only in more compact bioreactor sizes but also better ability to handle shock organic loadings as well as mitigate inhibition and toxic impacts. Other advantages include enhanced retention of biosolids, and better sludge settling characteristics which may affect other things such as the sizing of secondary clarifiers. The most commonly used fixed-film processes in wastewater treatment are trickling filters, and rotating biological contactors (RBCs) [15]. Anaerobic filters [16] and aerated biological filters [17] are also used, albeit less frequently than trickling filters and RBCs. When the biofilm is immobilized on particulate solid carriers in fluidized bed, the advantages of BFBs, such as increased liquid-solid interfacial area and enhanced contact between liquid and media, can be utilized to further increase the efficiency of the fixed-film bioreactors [18]. It should be noted that such biological fluidized bed fixed-film bioreactors are all operated in the conventional particulate fluidization regime, with relatively low liquid velocities.
These processes (suspended growth and fixed-film bioreactors including fixed-film BFB), however, can only achieve secondary effluent quality and additional treatment is required for nitrogen (N) and phosphorus (P) removals. To address the above problem and in response to increasingly stringent effluent nutrient criteria as a result of deteriorating surface water quality, biological nutrient removal (BNR) processes have become increasingly popular recently [19]. In BNR processes, nitrogen and phosphorus can be removed simultaneously [20, 21]. A typical BNR process is shown in FIG. 1. These BNR processes are essentially suspended growth systems, which employ a combination of anaerobic, anoxic, and aerobic suspended growth biological reactors with or without primary clarification. BNR processes involve diverse microbial groups and utilize the ability of selected microbes, known as phosphorous accumulating organisms (PAO) to undertake luxury phosphorus uptake, whereby some of the phosphorus stored in the cells is released during anaerobic conditions to be followed by a greater phosphorus uptake during aerobic conditions. Furthermore denitrifying bacteria that can elicit oxygen from nitrates reduce the organic loading to be treated aerobically.
BNR processes are known to offer several advantages over the more conventional activated sludge processes, namely superior effluent quality, a significant reduction in aeration energy requirements (likely due to utilization of formed nitrates to remove organic matter), improved sludge settling characteristics, a reduction in sludge quantities (likely due to lower bacterial yields in the anoxic tanks), and the elimination/minimization of chemical sludge. Although BNR activated sludge systems require more process controls as compared to conventional activated sludge systems, advances in process controls and data logging capabilities have significantly reduced human requirements and thus both systems now require comparable operator attendance. Consequently these BNR processes offer significant savings in both capital and operation/maintenance cost, in addition to the advantages of BNR over conventional activated sludge systems and their ability to meet stringent total nitrogen and phosphorus effluent criteria.
On the other hand, however, the reliability of the activated sludge BNR process in response to influent changes both in terms of quantity and characteristics (i.e. COD—Chemical Oxygen Demand, N, P, COD/P and COD/N ratios) have been questioned to the extent that many BNR plants have standby chemical dosing systems for P removal. Incomplete denitrification and low food to microorganisms (F/M) ratio have been observed to cause filamentous bulking conditions in BNR activated sludge systems [22, 23]. In some cases, external sources of carbon may be required to achieve P and N removal, because of low concentrations of readily biodegradable organics.
In view of the aforementioned shortcomings of the suspended growth BNR processes, there is a need to develop more effective BNR wastewater treatment processes. It would be ideal if the BNR concept can be combined with the fixed-film BFB process so that advantages of both processes can be utilized simultaneously. So far, no BFBs (without the combination with BNR) have accomplished effective phosphorus removal without using some chemical methods [24, 25]. To the best of our knowledge, a reliable BFB fixed-film BNR process that simultaneously achieves biological phosphorus and nitrogen removal has not been developed. Although nitrogen removal by simultaneous nitrification-denitrification has been accomplished in biological filters [26] by alternating oxic and anoxic conditions within the filter, very low nitrogen removal efficiencies (˜20% influent nitrogen) have been achieved.
It would be very advantageous to provide a high-efficiency biological fluidized bed for simultaneous removal of carbon, nitrogen and phosphate. However, one of the key difficulties is how to arrange the three different processes, anaerobic, anoxic and aerobic processes, in an integrated fluidized bed system.